1. Field of the Invention
The present invention relates to a rotation speed control circuit, rotation speed control method, and a fan system, and more particularly, to a rotation speed control circuit, rotation speed control method, and a fan system capable of stabilizing a rotation speed.
2. Description of the Prior Art
Computer systems are essential information tools for most people. For a personal computer, a laptop, or a server, etc., an operating clock of a central processing unit has become higher, as has heat emission, such that requirements of heat dissipation have become more essential as well.
Prior art computers utilize a fan with a steady rotation speed to dissipate heat. However, when a system voltage varies, or the fan is affected by a background pressure, i.e. air pressure, in a closed environment, the rotation speed may vary as well. Therefore, the prior art usually needs to utilize a rotation speed control circuit to perform negative feedback according to the rotation speed, so as to control the rotation speed and prevent the rotation speed from becoming too fast or too slow.
Please refer to FIG. 1, which is a schematic diagram of a conventional fan system 10. The fan system 10 includes a fan 102, a fan driving circuit 104, a rotation speed determination device 106 and a rotation speed control circuit 108. The rotation speed control circuit 108 controls the fan driving circuit 104 to drive a rotation speed RS of the fan 102 at a target rotation speed RS_Tar. The rotation speed control circuit 108 includes a rising edge trigger 110, an external RC pin 112, a pulse width modulation (PWM) signal generator 114 and a first-order integrator 116. In short, the rotation speed determination device 106 generates a rotation speed signal FG according to the rotation speed RS of the fan 102. The rising edge trigger 110 is triggered at a rising edge of the rotation speed signal FG, to start charging the external RC pin 112 until a charging voltage Vc equals a specific voltage. During the time the external RC pin 112 performs charging operations, the PWM signal generator 114 sets an outputted PWM signal Sig_PWM as off-time. The first-order integrator 116 performs integration according to the PWM signal Sig_PWM, to generate a driving voltage Vd, such that the fan driving circuit 104 drives the fan 102 according to the driving voltage Vd.
For further details, please refer to FIG. 2, which is a schematic diagram of the rotation speed signal FG, the PWM signal Sig_PWM and charging voltage Vc of the fan system 10 shown in FIG. 1. As can be seen from FIG. 2, since the charging time of the external RC pin 112 is fixed, i.e. the off-time of the PWM signal Sig_PWM is fixed, the on-time of the PWM signal Sig_PWM can be taken as a period of the rotation speed signal FG minus the fixed charging time. In such a situation, if the rotation speed RS equals the target rotation speed RS_Tar, the period of the rotation speed signal FG is fixed, and thus the on-time of the PWM signal Sig_PWM, i.e. the period of the rotation speed signal FG minus the fixed charging time, is fixed as well. Therefore, the driving voltage Vd integrated by the first-order integrator 116 is also fixed, such that the fan driving circuit 104 can maintain the rotation speed RS at the target rotation speed RS_Tar.
If the rotation speed RS is less than the target rotation speed RS_Tar, the period of the rotation speed signal FG increases, and thus the on-time of the PWM signal Sig_PWM, i.e. the period of the rotation speed signal FG minus the fixed charging time, increases as well. Therefore, the driving voltage Vd integrated by the first-order integrator 116 also increases, such that the fan driving circuit 104 can increase the rotation speed RS to reach the target rotation speed RS_Tar. Similarly, if the rotation speed RS is greater than the target rotation speed RS_Tar, the period the rotation speed signal FG decreases, and the on-time of the PWM signal Sig_PWM decreases as well, such that the driving voltage Vd decreases as well. Therefore, the fan driving circuit 104 can decrease the rotation speed RS to reach the target rotation speed RS_Tar. As can be seen from the above description, the rotation speed control circuit 108 can perform negative feedback to stabilize the rotation speed RS.
The above prior art method, however, needs to utilize the external RC pin 112 and the first-order integrator 116 which includes two resistors and one capacitor to generate the driving voltage Vd. Therefore, when a user intends to adjust the target rotation speed RS_Tar, the user needs to consider complex calculations for the first-order integrator 116 and the external RC pin 112, and thus can not intuitively modify the target rotation speed RS_Tar to meet system requirements. Thus, there is a need for improvement of the prior art.